1. Field of the Invention
Exemplary aspects of the present invention generally relate to an exposure device, and more particularly, to an image forming apparatus using the exposure device, such as a digital copier, a laser printer, and a laser facsimile.
2. Description of the Background Art
Recent electrophotographic image forming apparatuses such as copiers, laser beam printers, and facsimile machines form an image by converting electronic information into optical information. Based on the optical information, an exposure device employed in the image forming apparatus projects light against a photoreceptor serving as an image bearing member to form a latent image thereon. Then, the latent image is developed with toner and the like, forming a visible image, also known as a toner image.
Two types of exposure devices are known in the art. One is an optical scanning device including a combination of a light source and a light deflector such as a polygon motor. The other is an array light source device having light emitting devices arrayed in a line so as to expose an entire surface of the photoreceptor in a scanning direction all at one time.
Of the two types of exposure devices described above, the array light source device is advantageous for various reasons, including 1) a smaller exposure device, thus resulting in reduction of the size of the image forming apparatus as a whole, 2) a narrower beam diameter on the surface of the photoreceptor, thus resulting in a higher-quality output image, and 3) longer product life of the exposure device, thus resulting in a longer lifespan for the apparatus.
Although advantageous, there is a drawback in the array light source device in that a depth of beam at a focal position is narrow. More specifically, although the optical scanning device has a depth of beam (a depth corresponding to ±10% of the minimum diameter of the beam) of approximately 5 mm, by contrast the depth of beam of the array light source device is as small as ±20 to 30 μm. This difference in the depth of the beam appears as a difference in a degree of tolerance of focus under environmental variations, for example variations in temperature.
In particular, the number of light emitting sources in the array light source device is approximately 10 E+2 to 10E+3 times more than those in the optical scanning device. Consequently, the array light source device releases more heat as the exposure device, causing thermal expansion (thermal deformation) in the light source device due to not only the variations in the temperature but also self-heating. Thermal expansion of the light source device due to heat causes fluctuation of a distance between the array light source and a focusing lens, increasing the beam diameter on the photoreceptor which causes displacement of the focal position. As a result, the quality of the image deteriorates.
To address this problem, Japanese Unexamined Patent Publication No. 2003-066306 (JP-2003-066306-A) proposes a method for correcting displacement of a focal position due to fluctuations in temperature in an exposure device. The method includes providing a temperature measuring device in an exposure device and a control device for adjusting the focal position according to a value measured by the temperature measuring device, and adjusts the focus according to fluctuation in the temperature.
Disadvantageously, however, the number of parts in the exposure device increases, thereby complicating efforts to make the image forming apparatus at low cost.
In view of the foregoing, a device capable of preventing deterioration of an image due to environmental changes while reducing the cost by reducing the number of parts is needed.